Liquids can be preserved when they are frozen in bags or bottles made of plastic material and, if necessary, are vacuum dried. When these are sensitive liquids with organic components, freezing must take place as quickly and uniformly as possible, in other words, controlled to prevent cold damage. This applies, in particular, when these organic components are living cells, for example, bacteria suspensions. When freezing is uncontrolled, the wall of the cell and cellular tissue may be destroyed as a result of a severe ice crystal formation.
Such a bacteria suspension consists, for example, of 95% water and 5% bacteria. With uncontrolled freezing, the survival rate of these cells may drop to an unacceptable low degree. But liquids with other organic components, for example, albumen solutions, vitamin solutions and vaccines may be damaged by uncontrolled freezing. A proven method of freezing such liquids for the purpose of preservation is to conduct freezing by means of a low boiling liquified gas, as a rule nitrogen. With liquid nitrogen as refrigerant, the liquid, for example, in bags or ampules can be cooled very quickly to the desired freezing temperature so that there is, for example, little time for an extensive ice crystal formation. But some time is needed to freeze such a liquid specimen from the outside to the inside so that to some degree inevitable cold damage and concentration of components in the core of the liquid occur.
British Pat. No. 1,376,972 discloses a device which provides for a very gentle freezing of such a liquid, namely, liquid egg. With this device, drops of liquid egg are produced, led into a bath consisting of a low-boiling liquified gas from which they are removed in the form of frozen pellets. The liquid can be deep frozen extremely fast to the target temperature because the drop has a small volume. This is still enhanced by the direct heat exchange between liquid and cooling medium since separating intermediate walls between cooling medium and the liquid to be deep frozen are eliminated. The spherical shape of the drops results, moreover, in a ratio of liquid surface to volume which is optimum for uniform freezing. The drops are produced with a peristaltic pump by a periodic compression of flexible tubes filled with the liquid to be frozen so that drops of the liquid are expelled from the flexible tube and through nozzles are led into the refrigeration bath.
This type of drops production is cumbersome. The peristaltic pump is a relatively complicated and trouble-prone machine which requires a constant monitoring. Since it is placed in the immediate vicinity of the cooling medium bath, there is the risk that the nozzles ice up. When a certain pellet diameter must be absolutely maintained, there are also problems since with the known device only pellets having a somewhat same diameter can be produced. A controlled freezing or sensitive liquid requires, however, drops and pellets having a uniform size because identical and, therefore, controlled freezing conditions can only be realized with such uniform sizes. The known device also has only limited possibilities, for example, for varying the size of the drops via the throughput. But when different liquids having a different viscosity are to be frozen to pellets, it is often desirable that a certain pellet size is associated with a certain liquid.